Gallium antimonide (GaSb) thermophotovoltaic (TPV) cells have been fabricated for electric power generation using infrared radiation from hydrocarbon fired radiant heat sources. The market for these cells may be expanded dramatically if the cell fabrication cost is reduced. Currently, sixteen (16) GaSb cells are fabricated laid out on three (3) inch diameter single crystal GaSb wafers. The GaSb wafers are obtained by growing three (3) inch diameter GaSb single crystals. There are two primary contributors to cost. The first factor is the cost of the single-crystal wafer itself. The second factor is the cost of the wafer processing steps to obtain the final cells.
In the semiconductor industry, it is known that the device cost may be reduced by increasing the wafer diameter. That is because wafer processing costs are generally per wafer costs rather than per device costs. It is very desirable to increase the diameter of a GaSb wafer in order to reduce GaSb cell costs. For example, one might obtain 64 cells from a six (6) inch wafer at nearly the same cost that one obtains sixteen (16) cells on a three (3) inch wafer now. However, the largest diameter GaSb crystal grown to date is a four (4) inch diameter crystal. The growth of a larger crystal requires both new expensive equipment and considerable experimentation. It is desirable to obtain a large diameter low cost GaSb wafer that may be processed into high performance GaSb TPV cells.
In the solar photovoltaic field, it is known that silicon photovoltaic cells may be fabricated on coarse grain polycrystalline wafers. Although there is some degradation in performance, the polycrystalline silicon solar cells are commercially available. The polycrystalline silicon wafers are obtained from cast polycrystalline silicon ingots.
Fabricating high performance GaSb TPV cells using polycrystalline GaSb wafers is not readily apparent. First, GaSb is a different material than silicon with a different crystal structure. Second, there is no theory describing how grain boundaries effect photovoltaic cell performance. Lastly, a TPV cell is not a solar cell.
Within the TPV field, there is no empirical information relating to polycrystalline material. Starting from the solar cell case, theoretical arguments may be made both for and against polycrystalline materials. The argument against polycrystalline material is that TPV cell materials have lower band-gaps than solar cell materials (0.7 V instead of 1.1 V) and therefore, TPV materials generate lower voltages. So, any loss in cell voltage for TPV cells from grain boundary shunts is more significant and intolerable. The argument in favor of polycrystalline material is that TPV cells operate at much higher current densities and, therefore, shunting currents from grain boundaries are less significant. A need exists to fabricate high performance GaSb TPV cells using polycrystalline GaSb wafers.